Integrating Sphere-Based Nephelometer for UAS Applications
The goal of this DoE Phase II SBIR project was to build prototypes that demonstrate a novel concept for a highly compact, yet sensitive (< 1 Mm-1) integrating nephelometer measuring aerosol scattering at three wavelengths (blue, green, and red), that are capable of deployment on lightweight airborne platforms such as UASs. The Nephelometer design aimed for a compromise between different design criteria including size/weight, detection limit, power consumption, time resolution; there is no free lunch, improving performance in one criterion leads to penalties in others. Specifically, for current, state-of-the-art, photon-counting nephelometers, the detection limit is inversely proportional to the square root of the photon count, which therefore must be maximized to achieve a low detection limit below 1 Mm-1, the target value of the DoE SBIR solicitation. For comparison, particle-free air with only molecular (i.e., Rayleigh) scattering has a scattering coefficient of ~ 10 Mm-1 in the green. The photon count is proportional to detection volume, light intensity (i.e., photon density or actinic flux) therein, and integration time. However, a straightforward increase of any of these parameters leads to severe penalties in other design criteria especially for UAS deployment; increasing detection volume yields a larger, heavier instrument, increasing light intensity yields increased power consumption, and increasing integration time lowers the measurement time resolution. These tradeoffs have so far prohibited development of a compact, yet sensitive nephelometer with high time resolution. However, in our design, we have identified and mitigated a common shortcoming of current nephelometers; light is generated, passes through the detection volume a single time and is then discarded. Therefore, we utilized the transformative concept of enclosing the detection volume with a highly reflective cavity that recycles light many times, thereby proportionally increasing the photon count and enabling the novel nephelometer to achieve the target detection limit (<1 Mm-1) while simultaneously reducing size/weight, increasing time resolution, and limiting power consumption. We designed and build nephelometer prototypes based on our results from phase I with a multi-reflecting cylinder cavity, because the cylindrical cavity yielded a better performance by reducing truncation errors compared to a sphere, brought the reflecting light closer to measurement volume, thereby increasing measurement sensitivity, and reduced the total volume of the measurement cell. We consequently named our prototypes Integrating Cavity Nephelometer (ICN) to indicate the expansion and improvement from our initial integrating sphere approach. Testing of our approach showed considerable gains in light intensity and detection speed, such that these prototypes were able to measure scattering coefficients with a sensitivity of better than 1 Mm-1 in 1 s, thereby reaching the desired design goal. An additional benefit of our design is a considerable reduction in the size of an integrating nephelometer, without compromising its measurement ability. With a total weight of 3kg the prototypes are only half the maximum weight of the desired specification. During this Phase II SBIR project, we have demonstrated the concept of light multiplication with our novel, highly reflecting cavities enclosing the detection volume. In phase I, we had already demonstrated that improvements in light intensity by factors of 50 and 30 for the spherical and cylindrical cavity, respectively are possible. Furthermore, we developed a Monte Carlo simulation of nephelometer performance as function of design parameters including cavity size, shape, reflectivity, and detection volume. We experimentally modified these design parameters and were able to verify the performance of our Monte Carlo simulation over a wide range of design parameters, thereby greatly increasing our confidence in simulation predictions. These predictions now have been tested against a professional illumination software LightToolsTM that utilizes the actual design of an instrument by importing the computerized design drawings into its calculations. For our prototype design, we calculated a total light multiplication factor of 9.8, while the light tools evaluation yielded a near identical factor of 9.2. In Phase I, we realized that the coupling of the LED light into the cavity is a critical design parameter and with our optical design study, we were able to solve this issue by utilizing a light tube design. This allows with very little contact to the reflecting cavity while still transporting 80% of the light into the cavity. Unfortunately, we could not utilize an increase in sensitivity of a factor of 25 promised by our optical study through the use of a two-lens aperture as Photo Multiplier Tube (PMT) entrance. After building the ICN prototype we realized during the initial testing that we had a tremendous stray light problem with the PMMA lenses and we had to abandon this approach due to poor performance. We lost quite a bit of time due to this design detour, but we recovered quickly, because we had learned in Phase I that the concept of a baffle aperture and our ICN design was flexible enough to accommodate the needed design change. Within the scope of the project, we developed firmware and software to operate the ICN. The firmware is operating the ICN after previous parameter setup in a fully automatic mode. Data are stored on a removable memory storage card with sufficient capacity for even very long deployments. Software for aiding the user to set parameters has been developed as well as software to visualize the collected data in real time. The developed PCB electronic board has various access ports including ethernet, USB, and RS232 allowing to connect to the ICN. Utilizing a Ethernet configuration for UAS deployment, easily integrates the ICN into the network and, in case a remote connection allows to access the nodes directly, the software would allow to connect directly and to display live data even during flight, the access to all error and status information will further help to ensure the success of a mission. The ICN has two operation modes, the power saving conventional sequential measurement at three wavelengths and the Fast Fourier Transform (FFT) mode where all three wavelengths LEDs are operated simultaneously. This is enabled by power modulation of each light source at a different frequency and clearly separating these three signals in frequency space after Fourier transform of the detected scattering signal. This mode uses more power, but demonstrated clearly a dramatically increased detection speed. While the sequential mode does not measure the same ensemble of particles due to the fact that they move out of the detection volume before the next LED is engaged, the FFT mode operates LEDs a all 3 wavelengths at the same time and therefore measures the same ensemble of particles at the same time. The resulting Ångström exponents can thus be measured and attributed to the same sample, avoiding systematic errors in a dynamic environment. Additional issues addressed in Phase II include (1) the design and build of a dual stage diffusor inlet for the isokinetic sampling with the ICN inflight and the design, build and testing of lower flow PM2.5 cyclones for our ICN and other nephelometers.; (2) addressing the question how small can we get by investigating smaller photo detectors such as avalanche photo detectors (APD’s) and silicon photo multipliers (SiPM) and by utilized our Monte Carlo simulation to show a path toward further size reduction through “proportional shrinking”; (3) PM2.5 Federal Equivalency Method testing at four different locations with similar three wave length nephelometers to find out if the ICN would be able to gain USEPA FEM status, the tests are still ongoing, but with encouraging results; (4) the planning of flight tests with helicopters and the integration in light weight, class 1 UAWs; (5) the evaluation of the short term measurement ability for source apportionment, traffic studies etc., showing the exceptional abilities of our novel technology. In summary, Phase II was highly successful and has clearly demonstrated that our novel prototypes employing a highly reflective cavity for light recycling yielded a highly compact, yet sensitive (< 1 Mm-1) integrating nephelometer measuring aerosol scattering at three wavelengths (blue, green, and red). Such a nephelometer is ideal for deployment on lightweight airborne platforms such as UAS. The ICN still needs additional testing and hardening for extreme environments, but has already shown its exceptional potential. It will also be suitable for many other novel nephelometer applications, where currently nephelometer use is prohibitive due to performance limitations, thereby greatly increasing the nephelometer market size and vitality. Such applications will include monitoring of events such as wildfires and regulatory or exploratory measurements for plume distribution, plant emissions, processes surveillance and control, etc.
- Research Organization:
- American Ecotech
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0015031
- OSTI ID:
- 1580999
- Type / Phase:
- SBIR (Phase II)
- Report Number(s):
- DOE-Am. Ecotech-15031-2
- Country of Publication:
- United States
- Language:
- English
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